1. Field of the Invention
The present invention relates generally to electrophotographic printing and, more particularly, to a method of printing grey-scale images with a two-stage electrophotographic printer and to a novel toner for use with the method. Furthermore, the invention allows the conversion of a conventional two-color printer to a monochrome printer having extended grey-scale capabilities. Furthermore, the invention may be applied to a full-color, high-quality EP printer with extended grey-scale.
2. Description of the Prior Art
In recent years, electrophotographic (also called xerographic) printers have come to dominate the printer market. In a typical process, a photoconductive material is coated onto a drum or a belt, constituting a photoconductor. This photoconductor is provided with a uniform electrostatic charge in the absence of light. Then the coating is exposed to light by scanning a laser across the surface which imagewise discharges the uniform electrostatic charge, forming a latent electrostatic image which corresponds to the information to be printed. Similarly, the photoconductor may be exposed by other light sources such as light-emitting diodes.
The image is developed by being briefly contacted with a resin powder, called toner, whereupon a visible toner image is formed on the photoconductor, and said toner is then transferred to paper. The toner image is fixed to the paper by heating or by the action of solvents. In this way, an image which is resistant to abrasion, is obtained electrophotographically on plain paper.
The latent electrostatic image is contacted by toner in a development system. Development systems have been described which use one powder (toner), called monocomponent development systems, or which use two powders (toner and carrier), called magnetic brush development systems. In the magnetic brush development systems the carrier may be in the form of grains of polymer-coated or uncoated iron particles. The magnetic properties of the carrier particles are used to transport the carrier and toner particles to the electrostatic image. In the monocomponent systems the toner is transported to the electrostatic image either using magnetic forces (if magnetic material is incorporated within the toner) or using electrostatic forces (the toner's electrostatic adhesion to a roller). The material used for the toner may be a resin powder containing dyestuffs or pigments, e.g., carbon black. For both development system approaches the toner is charged by frictional electrification, either by contact against the carrier in the magnetic brush system or against a roller in the monocomponent system. When the toner is brought into contact with the electrostatic image, the charged toner particles are attracted by the electric field of the electrostatic image being developed.
Unfortunately, many electrophotographic printers do not render faithful or pleasing copies of continuous tone originals. The usual discharge characteristic of the photoconductive material and developability of the electrophotographic development system combine to yield a tone reproduction curve with a steep slope and few levels of darkness intermediate black and white. The result is a copy with washed out highlights and overdeveloped shadows. A standard technique used to improve the grey scale reproduction is to expose the photoconductor through a transmissive screen to produce a fine, spatially modulated voltage pattern on the photoconductive material. Development of this modulated pattern yields a tone reproduction curve with a lower slope and an extended range in the output. The result is a more faithful and pleasing copy.
A similar effect is found in the halftoning process employed in digital printing of pictures. The digital image is formed of a textured pattern of black and white spots, or pixels, which gives the impression of a grey when viewed at normal reading distance. If the halftone frequency and number of distinguishable grey steps are both sufficiently high, the printed picture will be pleasing to the eye. Digital halftone methods are often employed in binary electrophotographic printers. This is exemplified by laser printers with only two laser intensity levels: on and off. Since these digital printers produce individual black or white pixels, the opportunity exists to group the pixels into a halftone pattern to effectively produce spatially uninterrupted gradations of grey tones.
Groupings of pixels are commonly discussed in the literature of digital halftoning. Pixels can be grouped randomly (called dithering with white noise) or regularly. In regular arrays they can be grouped in clustered dots or dispersed dots. A well known grouping is called supercircle, where the dots are grouped regularly to form clusters in the form of ever-increasing larger circles as the grey level gets blacker.
A disclosure specific to electrophotography is U.S. Pat. No. 4,845,524 to Okamoto et al. This patent describes a two-step electrophotograhic process in which a black toner is used for a first step of transferring the image of an original document onto a white copy sheet. Thereafter, a white toner is used for a second step of superimposing a halftone solid onto a specified region of the image formed in the first step. The composition of a typical white toner said to have enhanced qualities is disclosed in U.S. Pat. No. 4,855,204 to Fujii et al. W. Lama, et al, in "Hybrid (Gray-Pixel) Halftone Printing", J. of Imaging Technology, Vol. 15, No. 3, June 1989 suggest that a significant improvement in pictorial quality can be achieved through a hybrid halftone technique where the pixels can have more than two levels (on-off). For instance, according to their concept, a trinary printer (black, white and one grey pixel level) can produce a vastly greater number of output grey steps than a binary printer. In a four-level printer having two intermediate grey pixel levels, the output steps can approach a continuum as viewed by the human eye. While the authors of the paper discuss the use of intermediate grey pixel levels, they do not disclose any method for physically achieving them, however.
It was in light of the foregoing that the invention has been conceived and is now reduced to practice.